Modification technology of a polymer is known as a technology for modifying characteristics of natural polymer such as natural rubber, and a synthesized polymer. For example, modification of a terminal structure to a synthetic rubber such as styrene-butadiene rubber, addition modification on a main chain, modification method in a polymerization step, and the like are proposed (see PTLs 1 to 7 listed below). Furthermore, because natural rubber is a natural product, a modification method of directly adding a functional group to a side chain or adding a functional group by grafting a polymer is proposed as a technology for changing the characteristics thereof (see PTLs 8 to 12 listed below).
Such a modified polymer is used as, for example, a rubber component in a rubber composition. Generally, in a rubber composition, a diene rubber is used as a rubber component, and a filler such as carbon black or silica is compounded with the rubber component. To improve properties of such a rubber composition, the modified polymer described above is used as a rubber component.
Various modification methods are proposed as above, but a modification method in which a functional group is easily incorporated in a main chain structure is not known, regardless of a solution polymerization or an emulsion polymerization.
PTL 13 listed below discloses a depolymerized natural rubber useful as an adhesive, a pressure-sensitive adhesive, a sealing agent, a caulking agent, a plasticizer and the like. In PTL 13, a liquid depolymerized natural rubber having a number average molecular weight of from 2,000 to 50,000 is produced by subjecting a deproteinized natural rubber dissolved in an organic solvent to air oxidation in the presence of a metal catalyst to depolymerize the deproteinized natural rubber. PTL 13 discloses that a main chain is decomposed by air oxidation to form a molecular chain having a carbonyl group at one terminal and a formyl group at other terminal, and the formyl group is recombined by aldol condensation. However, in PTL 13, the depolymerization is conducted in a solution of an organic solvent, and PTL 13 does not disclose that the recombination is performed by changing a system containing a decomposed polymer to basicity from acidity or to acidity from basicity. Furthermore, PTL 13 is to obtain a telechelic liquid rubber having carbonyl groups at both terminals, and has an object to obtain a liquid rubber by decomposing a natural rubber into low molecules. For this reason, in PTL 13, a polymer is not modified by recombining a main chain structure while controlling without extremely decreasing a molecular weight.
NPL 1 listed below discloses a composite of a polyester having a carbon-carbon double bond in a main chain and polyisoprene derived from a natural rubber by a main chain exchange reaction. However, the technique disclosed in NPL 1 is performed by an olefin cross metathesis reaction, and requires a metal catalyst such as Grubbs catalyst, and control of a reaction system is not generally easy.